From Russian morphonology: закалять and закаливать
The article considers the distribution of secondary imperfective verbs ending in -ывать/ ‑ивать and in ‑ать/-ять in standard Russian. Factors affecting the distribution are proved to be heterogeneous, referring to various language levels, and partly controversial. The basic principle, covering the bulk of cases, turns out accentual: the stress of derived imperfective verb patterns upon the long form of past passive participle of the base verb. All other principles appear secondary and can merely modify the basic one in particular cases. These additional principles fall into several types, such as: phonemic and morphonemic (regarding alternation of consonants), derivational and stylistic (determining formal vs informal styling). Finally, it is shown that this present-day distribution of imperfectives dates back to a relatively recent time.